FIG. 3 is a schematic longitudinal sectional view showing the general construction of an ordinary nuclear reactor. In the drawing, a reactor vessel 10 is equipped with a core vessel 12 in which a fuel assembly 14 is supported. The fuel assembly 14 in the core vessel 12 is surrounded by an upper core plate 16 on the upper side, by a lower core plate 18 on the lower side, and by a neutron reflector 20 at the periphery.
Next, the assembly structure of this neutron reflector will be described with reference to FIG. 4. The neutron reflector 20 is formed by vertically stacking together eight substantially annular stage portions, which are fastened by eight tie rods 22 that are circumferentially arranged. The positioning when assembling each stage portion is effected by a positioning pin 23. The lowermost stage portion 20A of the neutron reflector 20 has at the periphery thereof four flange portions 201 (only two of which are shown in the drawing); similarly, the top stage portion 20C has four flange portions 202 at the periphery thereof.
As shown in FIG. 5, the tie rods 22 extend through the neutron reflector 20 from the top stage portion 20C to the lowermost stage portion 20A. The lowermost end portions of the tie rods 22 are passed through the lower core plate 18 and screwed into a flange portion 13 formed in the core vessel 12. To the upper end portion of each tie rod 22, there is mounted a nut 24 for pressing down the upper surface of the top stage portion 20C of the neutron reflector. By turning these nuts 24, the neutron reflector 20 is tightened between them and the lower core plate 18, and is secured to the flange portion 13 of the core vessel 12 through the intermediation of the lower core plate 18.
The neutron reflector 20 has a large number of flow holes for cooling, through which cooling water flows. FIG. 6 shows a structure of inlet portions of the flow holes 204 in the lowermost stage portion 20A of the neutron reflector 20. In the drawing, plugs 181 are provided in the lower core plate 18 mounted to the flange portion 13 of the core vessel 12. Cooling water flowing in through these plugs 181 passes orifices 203 provided in the lowermost stage portion 20A of the neutron reflector 20 and flows upwards through the flow holes 204. These flow holes extend through the 8-stage neutron reflector 20 from the lowermost stage portion 20A to the top stage portion 20C, so that the cooling water flowing in through the orifices 203 of the lowermost stage portion 20A rises through the flow holes 204 to flow out through the flow holes of the top stage portion 20C of the neutron reflector 20.
Where the coolant water passes through the orifices 203 of the lowermost stage portion 20A, a pressure loss is generated, and, due to this pressure loss, a great lifting force is applied to the entire assembly structure of the neutron reflector 20. Most of this lifting force is generated when the cooling water passes through the orifices 203 of the lowermost stage portion 20A, and the force applied to the remaining seven stage portions, that is, from the second stage portion 20B to the top stage portion 20C is relatively small. In view of this lifting force, the eight tie rods 22 are fastened to thereby press the neutron reflector 20 against the lower core plate 18.
However, in a conventional structure, in which the neutron reflector 20 is pressed against the lower core plate 18 by the eight tie rods 22, when relaxation or loosening is generated in the tie rods 22 as a result of neutron irradiation, there is a possibility of the fastening force for pressing down the neutron reflector 20 against the lifting force falling short of the required level.